1. Field of the Invention
The present invention relates to an apparatus for decoding coded video data and particularly, to a coded video data decoder in which video data of e.g. MPEG2 coded format is decoded with a frame memory minimized in the storage size.
2. Description of the Related Art
Among various methods of minimizing the storage size of a frame memory in decoding of coded video data is a scalable decoder (FIG. 1) such as depicted in "Scalable decoder without low-frequency drift" by Iwahashi et al, (Singaku Giho, DSP94-108). The scalable decoder is a device for decoding a part of coded data and thus allows the storage size of a frame memory to be decreased with the use of IDCT (inverse discrete cosine transform) with a low degree as shown.
Video data of e.g. MPEG2 coded format is variable length decoded by a variable length decoder 1 and resultant quantized DCT coefficients are inverse quantized by an inverse quantizer 2 to produce 8.times.8 DCT coefficients. When 4.times.4 DCT coefficients in the 8.times.8 DCT coefficients are subjected to two-dimensional IDCT process of a 4.times.4 IDCT 10, video data consisting of 4.times.4 pixels is reconstructed which is 1/2 of the original video data in both the horizontal and vertical directions. If a block to be decoded is motion compensation blocks, the motion compensation blocks are supplied from a motion compensator 8 and added to the 4.times.4 pixel data for reproducing decoded video data. The decoded video data is 1/2 of the original size in both the horizontal and vertical directions and enlarged by an upsampler 11 before stored at the succeeding step in a sequence in a memory, not shown, for display.
Also, the decoded video data is stored in a frame memory 6 of which storage size is 1/4 of the original size because the data to be stored is 1/2 of the original size in both the horizontal and vertical directions. For carrying out a motion compensation process in a reduced space, a motion vector converter 12 is provided for converting a motion vector to 1/2 in both horizontal and vertical and extracting its corresponding blocks from the frame memory 6. If the two-dimensional IDCT process is executed with 4.times.4 IDCT in the horizontal direction and 8.times.8 IDCT in the vertical direction, the decoded video data and the frame memory are 1/2 of the original size.
The foregoing conventional method is capable of reducing the frame memory size by changing IDCT size but may create drift noise due to inconformity of a predictive image between the coding side and the decoding side and lower the quality of a reconstructed image. Also, the resolution of the reconstructed image will be as low as 1/2 or 1/4 of the original.